Eptazocine hydrobromide is an antagonistic analgesic agent and has been commercially available as an injection preparation where cancerous pain and postoperative pain are the indications.
Patent Document 1 mentions a process for producing a 4-benzoazonine derivative represented by the formula (I) which is an important intermediate for the production of eptazocine hydrobromide in which a perhydroazepine derivative represented by the formula (II) is subjected to a ring closure reaction with Lewis acid.

In the Patent Document 1, there are mentioned polyphosphoric acid, hydrobromic acid, etc. as the Lewis acid. However, there is a problem that, since the polyphosphoric acid is a viscous substance, its handling is bad and further that a purifying treatment after producing a ring-closed compound represented by the formula (I) is very difficult.
Accordingly, there has been a demand for a simple, easy and efficient process for producing a 4-benzoazonine derivative where no polyphosphoric acid is used.
In addition, when hydrobromic acid mentioned in the Patent Document 1 is used, the hydrobromic acid which is a volatile strong acid is heated to reflux whereby there is another problem that the reaction condition is severe, that a reaction container is apt to be corroded and that danger is accompanied therewith.
Therefore, it is desirous to avoid the use of hydrobromic acid in view of the health and the safety of persons belonging to the production and the protection of environments.